Force transducer etched from silicon

ABSTRACT

An accelerometer fabricated by silicon etching techniques. A first suspended beam is formed having a first conductive portion, the beam being deflectable in response to an acceleration force. A second beam having second and third conductive portions is suspended over the first beam. Phased lock loop circuitry oscillates the second beam at resonance and provides an electrical signal proportional to the acceleration force.

This is a division of application Ser. No. 07/319,495, filed March 6,1989 now U.S. Pat. No. 4,945,773.

BACKGROUND OF THE INVENTION

The field of the invention relates to force transducers wherein anapplied force, such as an acceleration force or a fluid pressure force,is converted into an electrical signal. In particular, the field of theinvention relates to force transducers and methods for producing suchtransducers by etching silicon substrates.

Force transducers are known having a suspended mass, such as a pendulumor cantilever, which deflects in response to an applied force.typically, capacitive plates are coupled to surfaces of both thesuspended mass and an adjacent structure. As the suspended massdeflects, the resulting change in capacitance provides an electricalindication of the applied force.

An example of an accelerometer is disclosed in U.S. Pat. No. 4,679,434issued to Stewart et al. A cantilever is formed by etching asubstantially U-shaped cavity through a silicon wafer. Two Pyrex™ planarsurfaces are then attached to opposing planar surfaces of the substratesuch that the cantilever is suspended therebetween. Conductive platesare bonded to both Pyrex™ surfaces and the suspended mass. Adifferential amplifier applies a voltage to the plates in response todetected deflection of the suspended mass for restoring the suspendedmass to its null position. The applied voltage is, allegedly,proportional to the applied force.

The inventor herein has recognized numerous disadvantages of the priorapproaches. In motor vehicle applications, for example, accelerometersare deployed in air bag systems wherein false triggering due to noise isintolerable. When the motor vehicle traverses rough road surfaces, theremay be deflection of the suspended mass due to noise resulting inerroneous interpretation as a collision. Since the prior approachesappear to have the ability of sensing displacement only in the timeframe of motion of the suspended mass, their ability to discriminateagainst vehicular noise is limited. An additional disadvantage is thatonly a portion of the structure disclosed may be fabricated by siliconprocessing technology. Thus, the potential advantages of batchprocessing technology are not fully utilized. An additional disadvantageis that deflection of the suspended mass is essentially arcuate.Accordingly, capacitive changes and corresponding electricalmeasurements are nonlinear with respect to the applied force.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a force transducerfabricated by silicon processing technology having high noise immunityand linearity.

The above object is achieved, and disadvantages of prior approachesovercome, by providing both an apparatus for detecting an applied forceand a method for fabricating the apparatus. In one particular aspect ofthe invention, the apparatus comprises: a first suspended beam etchedfrom a silicon substrate, the first suspended beam having a firstconductive portion and being deflectable in response to the appliedforce; a second suspended beam coupled to the substrate such that it issuspended opposite the first suspended beam, the second suspended beamincluding a second conductive portion and a third conductive portionboth being positioned opposite the first conductive portion; power meansfor applying electrical power between the first conductive portion andthe second conductive portion to oscillate the second suspended beam;and sensing means for sensing the applied force by sensing electricalpower between the third conductive portion and the first conductiveportion. Preferably, the power means includes a phased lock loop havingfeedback from the sensing means for oscillating the second suspendedbeam at its resonant frequency.

In another aspect of the invention, a pair of flexing means is includedwhich are symmetrically positioned around the first suspended beam forproviding linear deflection thereof in response to the accelerationforce.

By oscillating the second beam as claimed above, deflection of the firstsuspended beam is sampled at a rate faster than the deflection therebyproviding the advantage of higher immunity to noise than heretoforepossible. Thus, false triggering such as when traversing a rough roadsurface, is substantially eliminated. Further, by providing a firstsuspended beam coupled at opposing ends, beam deflection issubstantially linear with respect to an applied force. An advantage isthereby obtained of accurate sensing of an applied force.

In another aspect of the invention, a method for fabricating anapparatus for detecting an applied force is provided. More specifically,the method comprises the steps of: forming a sacrificial layer of anetchant material over one planar surface of a silicon substrate; forminga layer of polysilicon over the sacrificial layer; forming a firstsuspended beam by selectively etching the substrate; forming a secondbeam by selectively etching the polysilicon layer; etching away thesacrificial layer to suspend the second beam over the first suspendedbeam; forming conductive portions on both the first and the secondbeams; coupling an electrical oscillator between the conductive portionsfor oscillating the second beam; and coupling sensing circuitry betweenthe conductive portions for sensing the applied force by sensingcapacitive changes between the conductive portions.

The above aspect of the invention provides an advantage of fabricatingthe entire sensing apparatus by silicon processing technology.

BRIEF DESCRIPTION OF THE DRAWINGS

The above object and advantages may be better understood by reading theDescription of the Preferred Embodiment with reference to the followingdrawings wherein:

FIG. 1 is a perspective view of an embodiment which utilizes theinvention to advantage;

FIG. 2 is a cross-sectional view taken along line 2--2 of FIG. 1;

FIG. 3 is a bottom view of a portion of the embodiment shown in FIG. 1;

FIG. 4 is a block diagram of electronic circuitry coupled to theembodiment shown in FIGS. 1-3;

FIG. 5a is a cross-sectional view taken along lines 5a--5a in FIG. 1;and

FIGS. 5b-5h illustrate various processing steps in manufacturing theembodiment shown in FIG. 1 as viewed along lines 5a--5a in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The invention claimed herein will be better understood by reading anexample of a preferred embodiment, and a method for making theembodiment, described herein.

Referring to FIG. 1, in general terms which are described in greaterdetail hereinafter, force transducer 10 is shown in this examplefabricated from silicon substrate 12, {100} silicon in this example.Suspended beam 14 is shown integrally formed from substrate 12 andconnected at its opposing ends to supporting frame 16 via respectiveflexing members 18 and 20 which are also integrally formed from siliconsubstrate 12.

For the particular example presented herein, force transducer 10 isshown as an accelerometer wherein beam 14 deflects in response to anacceleration force. With suitable modification, such as addition of adiaphragm, beam 14 may also be responsive to other applied forces suchas fluid Pressure. In the later example, force transducer 10 may beutilized to advantage as a pressure transducer.

Flexing members 18 and 20 are shown symmetrically positioned around beam14 such that beam deflection is rectilinear in response to the appliedforce. The mass of beam 14 and dimensions of flexing members 18 and 20are chosen to provide a desired deflection and response time for aparticular application. For example, when used as an accelerometer in anair-bag system, it was found desirable to sense up to 50g's ofacceleration in 20ms. A corresponding deflection of 2um in 20ms was alsofound desirable. This deflection can be accommodated by a siliconsensing mass of 0.0012 grams supported by a flexing member havingdimensions of 40um×5735um×897um.

Suspended beam 14 includes conductive portion 24 (FIGS. 2 and 5a) whichforms a capacitive plate. In this example, conductive portion 24 isformed by doping a pattern of ions on top surface 26 of beam 24 asdescribed in more detail later herein. Other ways of producing a similarconductive portion are readily apparent to those skilled in the artincluding vapor deposition of a conductive layer such as nickel orplatinum. The conductive portion may also be formed by sequential vapordeposition of layers of chromium, gold, and chromium.

Conductive portion 24 is electrically connected to conductive tab 30 forcoupling to electronic circuitry 32. It is noted that conductive portion24 is connected to tab 30 by conventionally forming a hole into a dopedregion of substrate 12 and depositing a conductive material therein.This connection may also be accomplished by depositing conventionalconductive traces on substrate 12 between aconductive portion 24 and tab30.

Continuing with FIG. 1, and also referring to the cross-sectional viewshown in FIG. 2, suspended beam 36 is shown suspended over beam 14 andconnected to supporting frame 16. As described in greater detailhereinafter with particular reference to FIGS. 5a-5h, beam 36 isfabricated by growing, and appropriately etching, a layer of polysiliconover substrate 12. Those skilled in the art will recognize that beam 36may also be formed by other materials such as silicon oxide, or siliconnitride, or a hybrid combination of both. Further, beam 36 may be formedby bonding a silicon beam to substrate 12 utilizing a conventionalanodic bonding process.

Beam 36 is dimensioned such that it is electronically driven intooscillation at a desired frequency as described in greater detailhereinafter. For the particular example presented herein, beam 36 isoscillated at approximately 60 kHz which is achieved by beam dimensionsof approximately 1.6 um×2952 um×5795 um. In view of deflection by beam14, and oscillating amplitude of beam 36, a spacing between the beams of2um was found to be desirable in this particular example.

Reference is now made to the bottom view of beam 36 shown in FIG. 3.Sensing plate 40, driving plates 42_(a-d), and guard plates 44_(a-b) areshown formed on bottom surface 46 of beam 36. Plates 40, 42_(a-d), and44_(a-b) are formed by etching a doped polysilicon layer as describedlater herein. It is noted however that alternate methods of formingthese plates are utilized to advantage such as doping beam 36 in anappropriate pattern, or vapor depositing conductive layers such aschromium-gold-chromium on bottom surface 46. Forming the plates on a topor an intermediate surface of beam 36 may also be used to advantage.

Sensing plate 40 is shown coupled to conductive tab 64 on top surface 70of substrate 12 by conductive trace 50. Driving plates 42_(a-d) areshown interconnected and connected to conductive tab 66 on top surface70 by conductive trace 52. Guard plates 44_(a-b) are ohmically coupledthrough beam 36 to conductive tab 56 formed theron. Tab 56, andaccordingly guard plates 44_(a-b), are coupled to conductive tab 68 ontop surface 70 via conductive trace 58. Tabs 64, 66, and 68 areinterconnect electronic circuitry 32 with plates 40, 42_(a-d), and44_(a-b). As described in greater detail hereinafter, conductive traces50, 52, and 58, and conductive tabs 58, 64, and 66, are formed by vapordepositing layers of chromium-gold-chromium such that the traces extendalong bottom surface 46 of beam 36 and top surface 70 of substrate 12.

Force transducer 10 is hermetically sealed in a casing (not shown)filled with a gas at sufficient pressure to produce a desired damping ofmechanical oscillations.

A description of electronic circuitry 32 is now provided with referenceto the electronic schematic shown in in FIG. 4. Sensing capacitor 40' isa representation of the capacitance between sensing plate 40 andconductive portion 24. Similarly, driving capacitor 42' represents theelectrical equivalent of the capacitance between driving plates 42_(a-d)and conductive portion 24. Guard capacitor 44' represents thecapacitance between guard plates 44_(a-b) and conductive portion 18.Voltage source V_(s) is shown connected in series through resistor 70 toan input of isolation amplifier 72. Voltage source V_(s) is also shownconnected in series through resistor 70 and resistor 74 to one plate ofcapacitor 40', the other plate being connected to ground. Isolationamplifier 72 is shown as an operational amplifier having feedback to aninput terminal for stability. The output of amplifier 72 is showncoupled to one plate of guard capacitor 44' which has its other plateconnected to ground.

Oscillator 80 is shown coupled across driving capacitor 42' andresponsive to feedback signal f_(d) from detector 82 for driving beam 36into oscillation at its resonant frequency. Detector 82 is shown coupledacross resistor 74 for providing output signal M_(d) and feedback signalf_(d) which are proportional to deflection of beam 14'. The voltageapplied to sensing capacitor 40' through the series interconnection ofresistors 70 and 74 results in a current flow through resistor 74.Detector 82 detects changes in this current flow in response tocapacitive changes in sensing capacitor 40'.

Circuitry 32 forms a phased lock loop wherein beam 36 is driven atresonance and the amplitude of deflection is controlled by feedback tomaintain a constant amplitude of deflection. As beam 14 deflects inresponse to an acceleration force, there is corresponding change infeedback signal f_(d) for maintaining constant amplitude deflection andconstant frequency. There is also a corresponding change in outputsignal M_(d) which is proportional to deflection of beam 14 and,accordingly, the force of acceleration applied thereto. In oneparticular application, beam 36 is oscillated at 60kHz and beam 14 isdimensioned to respond at 12.5Hz. Thus, the phased lock loop provides,in this example, approximately 4800 samples per cycle of deflectionthereby achieving greater noise immunity than heretofore possible.

An example of fabricating accelerometer 10 utilizing conventionalsilicon processing techniques is now described with reference to FIGS.5a-5h. Referring first to FIG. 5b, a {100} silicon wafer 12 is shownhaving a conductive portion 24 patterned by conventionally doping ions,such as boron ions, through top surface 70. After the doping process,layer of silicon oxide 72 is grown over top surface 70 and layer ofsilicon oxide 74 is grown over bottom surface 76.

The process steps for suspending beam 14 from frame 16 and formingportions of flexing members 18 and 20 is now described with reference toFIG. 5c and FIG. 1. A masking pattern is first etched into silicon layer74 utilizing conventional photolithographic techniques to form openingsfor cavity 80, cavity 82, and pits 84, 86, 88, and 90. An anisotropicetchant, such as aqueous potassium hydroxide, is then applied to themasked openings in silicon oxide layer 74. The etchant acts against thenominal {100} planes at a rate approximately 40 to 100 times greaterthan the intersecting {111} planes thereby forming pits 84, 86, 88, and90. Concurrently, the etchant acts against the {100} planes to formcavities 80 and 82 extending from bottom surface 76 through top surface70 along {111} planes. Thus, the outer surface of the walls definingcavities 80 and 82 follow the {111} planes intersecting top surface 70at an angle of approximately 54.7°.

Footings 92 (FIG. 2), composed of silicon nitride, are formed intosilicon oxide layer 72 for isolating beam 36. In a conventional manner,footing openings are patterned and etched into silicon oxide layer 72. Asilicon nitride layer is then formed over top surface 70 andsubsequently etched such that only footings 92 remain of the siliconnitride.

Referring to FIG. 5d, layer of silicon oxide 94 is grown over bottomsurface 76 forming a passivation layer over pits 84, 86, 88, and 90. Afirst layer of polysilicon is then formed over silicon oxide layer 72and appropriately etched to form sensing plate 40, driving plates42_(a-d) and guard plates 44_(a-b) utilizing conventionalphotolithographic and etching techniques. The plates are also doped withions, such as boron ions, such that they are conductive.

As shown in FIG. 5e, second layer of polysilicon 96 is then grown overtop surface 70 covering plates 40, 42_(a-d), and 44_(a-b). Layer ofpolysilicon 90 is subsequently etched utilizing conventionalphotolithographic and etching techniques to form beam 36 as shown inFIG. 5f. Referring to FIG. 5g, silicon oxide layer 98 is then grown overtop surface 70 to form a passivation layer over beam 36.

Now referring to FIG. 5h, top layers of silicon oxide 72 and 98 areappropriately masked using conventional photolithographic techniques andetched to form etchant patterns on top surface 70 aligned with thedesired location of flexing members 18 and 20. An anisotropic etchant isthen applied to form etchant pits 102 and 104 through top surface 70thereby completing flexing members 18 and 20. It is noted that cavities80 and 82 may also be formed by partial etching from bottom surface 76and subsequent etching from top surface 70 when forming etchant pits 102and 104. In this manner, a more compact accelerometer 10 is achieved.The silicon oxide layers are then etched, in a manner known assacrificial etching, such that beam 36 becomes suspended over beam 14 asshown in FIG. 5a.

An alternate fabricating process is now described, with continuedreference to FIGS. 5a-h wherein plates 40, 42_(a-d), and 44_(a-b) areformed from conductive layers rather than doped polysilicon. The processsteps previously described herein with reference to FIGS. 5b-5c arerepeated. With reference to FIG. 5d, plates 40, 42_(a-d), and 44_(a-b),are formed by vapor deposition of sequential layers of chromium, goldand chromium over silicon oxide layer 72. These layers areconventionally patterned and etched to define plates 40, 42_(a-d), and44_(a-b). In addition, traces 50, 52, and 58, and tabs 56, 64, 66 and 68(FIGS. 1 and 2) are also defined during this step. The remaining processsteps are the same as those previously described herein with referenceto FIGS. 5e-5h.

Those skilled in the art will recognize that there are other processesutilizing silicon processing technology which may be utilized toadvantage to produce a force transducer embodying the invention such asthe one illustrated in FIG. 1. For example, conductive portion 24 couldbe formed by vapor deposition of a conductive material over top surface70. Similarly, plates 40, 42_(a-d), and 44_(a-b) could be formed byvapor deposition of a conductive material over the top surface of beam36. Conductive plates 40, 42_(a-d), and 44_(a-b) could also be formed byappropriately doping suspended beam 36. It is also noted that suspended36 may comprise other materials such as silicon oxide, silicon nitride,or a hybrid combination of both or other similar materials. Inconfigurations utilizing a hybrid combination, plates 40, 42_(a-d), and44_(a-b) could be formed between layers.

Suspended beam 36 may also be formed separately and bonded to substrate12. For example, in another alternate embodiment, beam 36 is formed byetching a separate silicon wafer and bonding the resultant beam tosupporting frame 16 by conventional anodic bonding techniques. In thislater example, conductive plates 40, 42_(a-d), and 44_(a-b) are formedby either doping silicon beam 36 or depositing conductive material, suchas nickel or platinum, on the underside of beam 36.

This concludes the description of the preferred embodiment. The readingof it by those skilled in the art will bring to mind many alterationsand modifications without departing from the spirit and scope of theinvention. For example, several alternate processes in constructing aforce transducer have already been described. Accordingly, it isintended that the scope of the invention be limited only by thefollowing claims.

What is claimed is:
 1. A method for fabricating an accelerometer fromsilicon, comprising the steps of:etching a pair of cavities through asilicon substrate from an exposed surface of said silicon substrate todefine a first suspended beam therebetween, said first suspended beambeing deflectable in response to acceleration; forming a firstconductive portion on said first suspended beam; bonding a silicon basedmaterial to an opposite surface of said silicon substrate; etching apair of cavities through said silicon based material bonded to saidsubstrate to define a second suspended beam suspended opposite saidfirst suspended beam; forming second and third conductive portions onsaid second suspended beam opposite said first conductive portion;coupling an electrical oscillator circuit between said first and secondconductive portions for oscillating said second suspended beam; andcoupling an electrical detector circuit between said first and thirdconductive portions for sampling electrical power between said first andthird conductive portions at sample times provided by said oscillationof said second beam.
 2. The method recited in claim 1 wherein said stepof forming said first conductive portion comprises doping said firstsuspended beam.
 3. The method recited in claim 1 wherein said siliconbased material is comprised of polysilicon and said step of forming saidsecond and third conductive portions comprises doping said polysilicon.4. A method for fabricating an accelerometer from silicon, comprisingthe steps of:forming a sacrificial layer of a silicon based materialreactive to an etchant over one planar surface of a silicon substrate;forming a layer of polysilicon over said sacrificial layer; etching apair of cavities from a surface opposite to said planar surface throughsaid silicon substrate to define a first suspended beam therebetween,said first suspended beam being suspended opposite said first suspendedbeam and deflectable in response to acceleration; forming a second beamby selectively etching parallel cavities through said polysilicon layerto define said second beam between said cavities; etching away saidsacrificial layer to suspend said second beam over said first suspendedbeam; forming a first conductive portion on said first beam; formingsecond and third conductive portions on said second beam; coupling anelectrical oscillator between said first and second conductive portionsfor oscillating said second beam; and coupling sensing circuitry betweensaid second and third conductive portions for sensing the applied forceby sensing capacitive changes between said conductive portions at sampletimes determined by said oscillation of said second beam.
 5. The methodrecited in claim 4 further comprising the step of forming flexiblemembers between said first suspended beam and said substrate.